Curable silicone compositions are used as sealing agents, protective agents, coating agents, or the like for optical semiconductor elements in optical semiconductor devices such as light emitting diodes (LEDs). Examples of such curable silicone compositions include curable silicone compositions comprising: a straight-chain organopolysiloxane having at least two alkenyl groups and at least one aryl group in a molecule; a branched-chain organopolysiloxane having an alkenyl group and an aryl group; an organopolysiloxane having at least two silicon atom-bonded hydrogen atom in a molecule; and a hydrosilylation reaction catalyst (see Patent Documents 1 to 3).
In general, such a curable silicone composition is used by blending a phosphor to convert the emission wavelength from the LED; however, because of poor dispersibility of the phosphor, the phosphor aggregates, thereby causing a problem in that the light from the LED becomes uneven.
Meanwhile examples of curable silicone compositions that form cured products having suitable hardness and strength include a curable silicone composition comprising: a straight-chain organopolysiloxane having at least two alkenyl groups in a molecule; at least two types of branched-chain organopolysiloxanes having a mass average molecular weight that is different from one another and comprising SiO4/2 units, R2R′SiO1/2 units, and R3SiO1/2 units (in the formula, R represents a monovalent hydrocarbon group having no aliphatic unsaturated bond, and R′ represents an alkenyl group); an organopolysiloxane having at least two silicon atom-bonded hydrogen atoms in a molecule; and a hydrosilylation reaction catalyst (see Patent Document 4).
However, the cured product of this composition has high gas permeability, and when this composition is used as a sealing agent for an optical semiconductor element, electrodes and phosphor thereof are corroded due to its low barrier properties to sulfur and water. As a result, there is a problem in that the light extraction efficiency from LED is reduced.